1. Technical Field
The disclosed embodiments relate to local oscillators, and more particularly to local oscillators in RF communication devices.
2. Background Information
RF communication devices such as cellular telephones generally employ local oscillators. The receiver circuitry of a cellular telephone may, for example, employ a first local oscillator to generate a first reference signal of a specific frequency. The frequency of the first reference signal is changed in order to tune the receiver of the cellular telephone to a desired operating frequency or to change operating frequency bands. Similarly, the transmitter circuitry of the cellular telephone may employ a second local oscillator to generate a second reference signal of a specific frequency. The frequency of the second reference signal is changed in order to set the transmitting frequency with which the cellular telephone transmits.
A circuit called a phase-locked loop (PLL) is often used realize such local oscillators within cellular telephones. Within the control loop of the PLL, a voltage controlled oscillator (VCO) generates a signal whose frequency is determined by a voltage signal supplied to the VCO. The quality of the signal output by the VCO generally varies depending on certain factors including the size of the components that make up the VCO and the power consumption of the VCO. For example, if the VCO is operated at a higher power then the VCO may introduce less phase noise into the VCO output signal. If, on the other hand, the VCO is operated at a lower power then VCO may introduce more phase noise into the VCO output signal.
The amount of phase noise that is tolerable is not constant but rather varies depending on a number of factors. If the cellular telephone receiver is operating in an environment that has a source of interference that is close in frequency to a signal to be received, then the receiver requires that the VCO output signal have less phase noise. If there is no interference source, then the requirements on VCO output signal quality are relaxed. In a Wideband Code Division Multiple Access (WCDMA) communication system, for example, the cellular telephone may be receiving at the same time that it is transmitting. To prevent the signal being transmitted from interfering with the signal being received, a receiver VCO output signal having low phase noise is required. In other operating modes of the WCDMA system in which the transmitter is not on, it is acceptable for the receiver VCO output signal to have more phase noise. When no strong interference sources are present, a lower quality VCO output signal may be adequate.
The VCO circuits within a cellular telephone may consume a considerable amount of power. In a mobile communication device such as a cellular telephone, extending the amount of time that the cellular telephone can operate on a single battery charge is desirable. U.S. Pat. No. 7,151,915 discloses determining a signal-to-noise ratio (SNR) of a signal being received onto an RF communication device. If the SNR is found to exceed a threshold, then the bias voltage supplied to the receiver's VCO is reduced such that the VCO is set into in a lower power consumption mode. If, however, the SNR is found to be degrading over time, then the bias voltage supplied to the receiver's VCO is increased and the VCO is operated in a higher power consumption mode. Accordingly, VCO power consumption is set at or near its lowest acceptable level for detected RF channel conditions.